1. Field of the Invention
The present invention generally relates to a code division multiple access (CDMA ) system, and more particularly to a multimedia direct-sequence code division multiple access (DS-CDMA) system which utilizes fully distributed uplink power control method.
2. Description of the Related Art
In a CDMA system, a plurality of mobile terminals share the same frequency band to communicate with a single basestation. Therefore, for example, if mobile terminals A and B transmit modulated signal waves to the basestation, the signal (not desired to be received) transmitted by the mobile terminal B interferes with the signal (desired to be received) transmitted by the mobile terminal A, and the communication of the mobile terminal A with the basestation is obstructed. The degree of interference depends on the received level of a signal (not desired to be received) at the basestation. If the degree of interference becomes large to certain level or more, communication between the mobile terminal and basestation becomes impossible.
If the transmission power of each mobile terminal can be controlled to always limit the signal level received at the basestation to a minimum necessary reception power, it becomes possible to maximize the number of channels capable of being communicated by the basestation. The more the transmission power shifts from the minimum necessary reception power, the less the number of channels capable of being communicated by the basestation.
As to transmission power control techniques of CDMA mobile communication, an IS-95 transmission power control method is known and described in TIA/EIA/IS-95 which is a standard system of digital cellular phones adopted in North America. The IS-95 transmission power control method will be described as follows.
Since two-way communication is essential for cellular phones system, a pair of an uplink traffic channel and a downlink traffic channel are used for the communication between the basestation and a mobile terminal. The uplink traffic channel is a channel for transmitting data from a mobile terminal to the basestation, and a downlink traffic channel is a channel for transmitting data from the basestation to the mobile terminal.
Referring to FIG. 1, it shows the structure of a mobile communication network. A public switched telephone network (PSTN) 200 is connected to a fixed terminal 201 such as a telephone and a mobile communication network 202. The mobile communication network 202 is connected to a plurality of basestations 203a, 203b, . . . Each basestation 203 communicates with mobile terminals 204a, 204b, . . . in its service area (cell) via radio channels 205.
Referring to FIG. 2, it graphically illustrates a CDMA-based communication system comprising a mobile station 10, a basestation 20, an uplink (reverse link) 30 which represents the electromagnetic wave communication link transmitted from mobile station 10 to basestation 20 and downlink (forward link) 40 which represents the electromagnetic wave communication link transmitted from basestation 20 to mobile station 10.
One method to control the level of interference in a CDMA-based communication system is by way of power control, that is, the power transmitted from the mobile station 10 to the basestation 20 (uplink, reverse link) or from the basestation 20 to the mobile station 10 (downlink, forward link) is controlled or varied. Power control on the uplink (reverse link) is required because of the so-called “near/far” problem. The problem occurs when a mobile station close to a basestation undergoes or incurs relatively low power propagation loss, and thus the basestation sees a very high level of power from the mobile station, whereas a mobile station located farther out undergoes relatively high propagation loss, and thus the power received by the basestation from the mobile station is very low. Power control for the reverse link is thus used to command the nearby and far-out mobile stations to reduce or increase the transmit power to the basestation, as is well known in the art.
U.S. Pat. No. 6,216,010 issued to Edwards et al., entitled “Up-link power control for fixed wireless access”, discloses that up-link power control for fixed wireless access communication networks. In the invention, a radio communications uplink power control system wherein distance information between an outstation and a base station and information relating to channel loss is employed by a power control algorithm whereby the signal power transmitted can be optimally controlled. The present invention provides a power control strategy which adapts not only to the total channel loss, but separately to the components of this loss. The total channel loss is calculated from the difference in the received signal power and the transmitted signal power (which is encoded in the message, or sent in addition to the message and therefore known to the receiving station).
U.S. Pat. No. 6,307,844 issued to Tsunehara et al., entitled “CDMA Communication System and Its Transmission Power Control Method”, discloses that an uplink channel transmission power control method for a CDMA mobile communication system performing one way communication. A base station measures the received level of data transmitted from each mobile terminal at each channel, and generates a transmission power control signal of each uplink traffic channel. The generated transmission power control signals are multiplexed, and the multiplexed common transmission power control signal is transmitted to all mobile terminals by using the common channel shared by the mobile terminals. Each mobile terminal derives the transmission power control signal of the uplink traffic channel used by the terminal, from the received common transmission power control signal, and controls the transmission power of a data packet.
In addition, power control schemes can be centralized or distributed depending on the nature of control. The basestation has the full knowledge of link gains for the centralized control to decide the power adjustment of each individual mobile station, but has only the local information of received power (signal-to-interference, SIR, for example) for distributed mechanism. Most literature investigated the distributed power control methods for FDMA/TDMA systems has focused on the study of the convergence speed of power control process under the assumption of same required SIR threshold for all mobile stations.
Paper issued by Qiang Wu, entitled “Optimum Transmitter Power Control in Cellular Systems with Heterogeneous SIR Thresholds”, IEEE Trans. On Vehicular Technology, pp. 1424-1429, Vol. 49, No. 4, July 2000. Qiang Wu first proposed a way which is extended from his results to determine the feasible solution of centralized power control for the CDMA systems, and he further proposed a distributed power control algorithm for the multimedia DS-CDMA system with different SIR requirement thresholds.
For the multimedia DS-CDMA systems with different SIR requirement threshold, the power adjustment formula of the distributed power control algorithm proposed in Wu reference is:       P    i          (              n        +        1            )        =            c      n        ⁢                   ⁢                  β        i                    γ        i                  (          n          )                      ⁢                   ⁢          P      i              (        n        )            where   P  i      (    n    )  is the transmitted power of mobile station i at the nth iteration, βi is the minimum SIR requirement threshold at the basestation for mobile station i,   γ  i      (    n    )  is the received SIR of mobile station i at the nth iteration, c(n) is the power convergence factor and       1          c              (        n        )              =      P    max          (      n      )      is the maximum transmitted power of mobile stations in all the cells at the nth iteration. At each iteration, mobile station i shall adjust its power by       c          (      n      )        ⁢           ⁢            β      i              γ      i              (        n        )            which value is informed by the basestation. From the numerical results, it is shown that the received SIR can converge by such algorithm. However, there is a drawback in such algorithm. It is impractical for the basestation to get the value of   P  max      (    n    )                  max unless it is informed via uplink signaling and exchanged among all cells.        
However, to carry out the power control process proposed in past, the basestation needs to know the maximum transmitted power of all the mobile stations in all cells and it is not practical for distributed system.
Accordingly, there exist the needs for providing a transmission power control method for a CDMA communication system which only needs the local measurements at the basestation and meets different SIR requirements of mobile stations based on fair principle.